The present invention relates to a semiconductor memory device, and more particularly to a tRAS adjusting circuit for extending an active operation in a self-refresh mode.
As generally known in the art, a dynamic random access memory (DRAM) is a volatile memory device including a plurality of cells, each of which contains one transistor and one capacitor.
Since the charge stored in a memory cell capacitor is drained as junction leakage current of a storage node and data are destroyed, a refresh operation must be periodically performed to maintain data stored in the memory cell.
A semiconductor memory device supports a self-refresh mode so as to maintain data stored therein. In the self-refresh mode, all input pins except for a clock enable pin are deactivated, and not only a refresh address but also a refresh signal is generated in the semiconductor memory device.
When entering a self-refresh mode, the semiconductor memory device repeatedly performs an active operation and a precharge operation therein according to a refresh cycle. Herein, the active operation refers to a process of selecting a word line and amplifying cell data, and the precharge operation refers to a process of isolating a memory cell having restored cell data and initializing a bit-line sense amplifier so as to enable the next active operation.
Meanwhile, in the conventional semiconductor memory device for low-power mobile goods, when a refresh operation is performed in a self-refresh mode, a precharge operation may be performed before memory cell data is sufficiently amplified by a bit-line sense amplifier.
That is, in the conventional semiconductor memory device for low-power mobile goods, a longer time than that for a normal operation is required for a bit-line sense amplifier to sufficiently amplify cell data in a self-refresh mode. However, since the conventional semiconductor memory device for low-power mobile goods establishes tRAS, which determines a precharge execution start time, to a level equal to that for a normal operation, data stored in a cell memory may be lost.